Suppression of bipolar conduction via bandgap engineering for enhanced thermoelectric performance of p-type Bi_0.4Sb_1.6Te₃ alloys

Substitutional doping is known to be effective when used to enhance the thermoelectric figure of merit zT, and this is generally explained as resulting from a reduction in the thermal conductivity caused by an additional atomic-scale defect structure. However, a comprehensive analysis of the substitutional doping effect on the electrical and thermal properties together has not been undertaken, especially when the bipolar thermal conductivity becomes serious. A previous study by the authors also showed that the zT of Bi_0.4Sb_1.6Te₃ thermoelectric alloys was enhanced by indium (In) doping due to the reduction of the total thermal conductivity. Here, we more closely analyze the electrical and thermal transport properties of a series of indium (In)-doped p-type Bi_0.4Sb_1.6-xIn_xTe₃ (x = 0, 0.003, 0.005, 0.01) using both the single-parabolic-band model and the Debye-Callaway model in an effort to investigate the origin of the observed thermal conductivity reduction more closely. The bipolar contribution to the total thermal conductivity was estimated exclusively based on a two-band model based on a single-parabolic-band model. Furthermore, the lattice thermal conductivity was calculated using the Debye-Callaway model while taking additional In substitutional defects into consideration. The calculations indicated that the significant suppression of bipolar thermal conductivity was achieved as a result of the increased bandgap in Bi_0.4Sb_1.6Te₃ caused by In doping. Additional point defects from In doping also reduced the lattice thermal conductivity, but not as much as the bipolar thermal conductivity did. The study suggests that the suppression of bipolar conduction by means of a bandgap modification can be an effective approach for enhancing zT further via a simple In-doping process in Bi_0.4Sb_1.6Te₃.